Fix Issue 7547

[craffael]

PR checklist

• [x] Read the contribution guidelines.
• [x] Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
• [x] Filed the PR against the correct branch: 3.0.0 branch for changes related to OpenAPI spec 3.0. Default: master.
• [x] Copied the technical committee to review the pull request if your PR is targeting a particular programming language. @mandrean @jimschubert

Description of the PR

I've adapted the api.mustache template with an additional "if" statement: If the response is a file response, use Nancy.Response. On top of that, I've refactor the computation of the return type into the returnType.mustache file in order to avoid (existing) code duplication.